Biological treatment of wastewater for removal of dissolved organics is well known and is widely practiced in both municipal and industrial plants. This aerobic biological process is generally known as the “activated sludge” process in which microorganisms consume the organic compounds through their growth. The process necessarily includes sedimentation of the microorganisms or “biomass” to separate it from the water and thus the final effluent with reduced Biological Oxygen Demand (BOD) and TSS (Total Suspended Solids) is obtained. The sedimentation step is typically done in a clarifier unit. Thus, the biological process is constrained by the need to produce biomass that has good settling properties. These conditions are especially difficult to maintain during intermittent periods of high organic loading and the appearance of contaminants that are toxic to the biomass.
Membranes coupled with biological reactors (MBRs) for the treatment of wastewater are well known, but are not widely practiced yet. In these systems, ultrafiltration (UF), microfiltration (MF) or nanofiltration (NF) membranes replace sedimentation of biomass for solids-liquid separation. The membrane can be installed in the bioreactor tank or in an adjacent tank where the mixed liquor is continuously pumped from the bioreactor tank and back producing effluent with much lower total suspended solids (TSS), typically less than 5 mg/L, compared to 20 to 50 mg/L from a clarifier. More importantly, these MBRs de-couple the biological process from the need to settle the biomass, since the biomass separation from the water is achieved by membrane. This allows operation of the biological process at conditions that would be untenable in a conventional system including: 1) high MLSS (bacteria loading) of 10-30 g/L, 2) extended sludge retention time, and 3) short hydraulic retention time. In a conventional system, such conditions could lead to sludge bulking and poor settleability.
The benefits of the MBR operation include low sludge production, almost complete solids removal from the effluent, effluent disinfection, combined COD, solids and nutrient removal in a single unit, high loading rate capability, no problems with sludge bulking, and small footprint. Disadvantages include aeration limitations, membrane fouling, and membrane costs.
Membrane costs are directly related to the membrane area needed for a given volumetric flow through the membrane, or “flux.” Flux is expressed as liters/m2/hour (LMR) or gallons/ft2/day (GFD). Typical flux rates in MBRs vary from approximately 10 LMH to about 20 LMH. These flux rates are relatively lower compared to those observed in drinking water applications (>50 LMH) with membranes having similar pore size and chemistries. These lower flux rates are mainly due to fouling of the membranes, and are the main reason for slower growth of MBR systems for wastewater treatment.
The MBR membrane interfaces with so-called “mixed liquor” which is composed of water, dissolved solids such as proteins, polysaccharides, suspended solids such as colloidal and particulate material, aggregates of bacteria or “flocs”, free bacteria, protozoa, and various dissolved metabolites and cell components. In operation, the colloidal and particulate solids and dissolved organics deposit on the surface of the membrane. Colloidal particles form a layer on the surface of the membrane called a “cake layer.” Cake layer formation is especially problematic in MBRs operated in the “dead end” mode where there is no cross flow; i.e., flow tangential to the membrane. Depending on the porosity of the cake layer, hydraulic resistance increases and flux declines.
In addition to the cake formation on the membrane, small particles can plug the membrane pores, a fouling condition that may not be reversible. Compared to a conventional activated sludge process, floe (particle) size is reportedly much smaller in typical MBR units. Since MBR membrane pore size varies from about 0.04 to about 0.4 μm, particles smaller than this can cause pore plugging. Pore plugging increases resistance for permeation through membrane and decreases flux.
In addition to these physical fouling mechanisms, the soluble polysaccharides (from “Biopolymer”) adsorb on the membrane surface as well as on the pore walls and form a slimy layer, thus contributing significantly to the total resistance for water permeation. It is known in the literature that extra-cellular polysaccharides secreted by bacteria include both anionic (e.g. uronic acids) as well as nonionic oligo and polysaccharides (e.g. hexoses and pentoses). Conditioning the mixed liquor with cationic, amphoteric or zwitterionic polymers results in complexation of only charged polysaccharides. The nonionic oligo/polysaccharides still form a slimy layer on the membrane surface, resulting in increased resistance for permeation.
Therefore, there is a need to develop improved methods of conditioning the mixed liquor in MBR systems to also address the fouling caused by nonionic oligo/polysaccharides and/or nonionic organic molecules, and increase the flux of the membranes.